I have DEM (cellsize 10×10 and cells.dim 450×300) and I need to create 100 random error grids that have the same dimensions as the DEM. The values in the error grids are generated from a normal distribution that has mean of zero and a standard deviation equal to the RMSE of original DEM ~(0, +-3). Then I need to add those error grids to the original DEM and create 100 realisation of the original DEM.